Image sensor arrays typically comprise a linear array of photosensors which raster scan an image-bearing document and convert the microscopic image areas viewed by each photosensor to video image signals. Following an integration period, the image signal charges formed on the photosensors are amplified and transferred as analog video signals to a common output line or bus through successively actuated multiplexing transistors.
For high-performance image sensor arrays, one embodiment includes an array of photosensors of a width comparable to the width of a page being scanned, to permit imaging generally without the use of reductive optics. In order to provide such a “full-width” array, however, relatively large silicon structures must be used to define the large number of photosensors. In one design, an array is made of twenty silicon chips, butted end-to-end, each chip having 248 active photosensors spaced at 400 photosensors per inch. Typically, the chips which are butted to form the array are formed in a silicon wafer, which is then “diced” into a number of chips, which are individually tested and then arranged into a bar shape.
In a sensor array of this design, each of the chips is a separate integrated circuit. Typically, each chip has its own individual video output, for the downloading of image signals ultimately from the photosensors on that particular chip. When an image is being scanned, video signals are output from each chip at a very high rate as the original hard-copy image moves past the linear array of photosensors on the chip. Thus, if the intended resolution of the chip in a bar is 400 spots per inch, a line of video data must be output from the chip every time the original image moves 1/400th of an inch.
An important concept with digital image scanners is “integration time.” The integration time is analogous to the opening and closing of the shutter of a film camera: it is the time period in which light from an image to be recorded is received. In the context of recording digital images, an integration time begins when a photosensor is set at a predetermined reference charge level and light impinging on the photosensor is permitted to influence the charge; the integration time ends when received light no longer influences the photosensor and the final charge is loaded out of the photosensor as a video signal. U.S. Pat. No. 5,148,168 gives a description of one embodiment of an image sensor in which three sets of photosensors, one for each primary color, are used to record a full-color image. U.S. Pat. No. 5,519,514 explains why precise control of the integration times of each of the three primary-color sets of photosensors is important to resulting image quality in a hard-copy scanner.